On-board systems of current motor vehicles comprise a generator formed as a three-phase generator with downstream rectifier circuit as an energy convertor, one or more energy storage units, for example batteries, and also electrical consumers to be supplied via the on-board system, said consumers being referred to together as system load. The three-phase generator, one or more energy storage units, and also the electrical consumers are referred to together as a first energy system.
The on-board systems also comprise further energy storage units, such as double-layer capacitors, which together are referred to as a second energy system.
The two energy systems have different nominal voltages as system voltages and are isolated from one another, but are also non-isolated.
The first energy system, for example with one or more 12 V batteries as an energy store, is used as an energy or power source for the normal consumers of the motor vehicle, such as lights.
The second energy system, for example comprising double-layer capacitors (ultracaps), is used as an energy or power source for the high-current consumers, such as motor starters.
The current flow between these two energy systems is controlled by a device, wherein this device is supplied with an operating voltage by an energy store, that is to say power source, of one of the two energy systems (generally by the 12 V battery of the first energy system).
In an on-board system, it is very important to achieve a largely safe system state (safe-state) in a nominal operating condition of the on-board supply, but also in a non-nominal operating condition of the on-board supply.
One of the requirements when observing a safe system state in the event of the occurrence of a system fault in the on-board system, in particular in the first energy system, which leads to a drastic voltage drop in the first and/or in the second energy system, is to hold closed the electrically closed current path in the first energy system in order to avoid an undesired parasitic current injection from the first energy system to the second energy system, which (the undesired parasitic current injection) leads to an uncontrolled overcharging of the energy store of the second energy system and also to overheating of the electronic components located in the current path from the first energy system to the second energy system.